1. Field of the Invention
This invention relates to control circuits for performing tracking servocontrol in an optical reproducing apparatus by which information recorded on a record medium, such as a video disc or digital audio disc, is optically reproduced.
2. Description of the Prior Art
In the field of information reproducing apparatus for reproducing a video signal, an audio signal and so on from a disc-shaped record medium which has thereon a record track formed with an alignment of pits provided in response to the information, there has been proposed an optical reproducing apparatus in which a light beam, such as a laser light beam, is used for scanning the record track on the disc-shaped record medium to read the information recorded therein by detecting the variations caused in the light beam, in a video disc system or a digital audio disc system.
For such an optical reproducing apparatus, it is required to perform automatic control for maintaining the light beam, which is directed onto the record medium to scan the record track thereon, in correct tracking relation to the record track. This automatic control is called tracking servocontrol and is indispensable for the optical reproducing apparatus of the kind mentioned above.
In order to carry out such tracking servocontrol, the optical reproducing apparatus is provided with a tracking servocontrol circuit which is operative to produce a tracking error signal representing positional deviation of the light beam from the center of the record track on the record medium. The tracking error signal is generated in response to the output signal of a photosensing device. The light beam is directed through an optical head to the record medium, which modulates the light beam in intensity and reflects the modulated light beam onto the photosensing device. The tracking error signal is employed; to drive an optical element, for example, a lens or a mirror constituting partially the optical head, to control the position thereof.
There has been proposed an optical system containing such a tracking servocontrol circuit for use in the optical reproducing apparatus as shown in FIG. 1. In the optical system of FIG. 1, a disc 1 has a spiral record track formed with an alignment of pits. A laser light beam emitted from a laser light source 2 enters through a diffraction grating 3 and a collimating lens 4 into a beam splitter 5 to pass through the same. The laser light beam having passed through the beam splitter 5 enters through a quarter-wave plate 6 into an object lens 7 to pass through the same to be caused to focus thereby on the disc 1. The object lens 7 is mounted movably in the direction transverse to the record track on the disc 1 and in the direction perpendicular to the surface of the disc 1 by driving devices 8 and 9, respectively. The laser light beam caused to impinge on the disc 1 is modulated in intensity in accordance with the record track thereon and then reflected to enter through the object lens 7 and the quarter-wave plate 6 into the beam splitter 5 to be reflected to the right in FIG. 1. The laser light beam reflected by the beam splitter 5 is led through a lens portion 10 to a photodetector 11. The photodetector 11 comprises plural light detecting elements and each of the light detecting elements detects the reflected laser light beam, which is modulated in intensity in accordance with the record track, from the beam splitter 5, that is, a reading light beam and produces a signal in response to the variations in intensity of the reading light beam. The signals derived from the photodetector 11 are supplied to a signal processing circuit arrangement 12 containing therein a tracking servocontrol circuit. In the tracking servocontrol circuit contained in the signal processing circuit arrangement 12, a driving signal for moving the object lens 7 to control the position thereof for performing tracking servocontrol is produced to be supplied to the driving device 8 provided for tracking servocontrol. The signal processing circuit arrangement 12 also contains a focus servocontrol circuit for performing focus servocontrol for maintaining correct focus of the laser light beam caused to impinge on the disc 1 in addition to the tracking servocontrol circuit, and further a reproduced information signal is derived from the signal processing circuit arrangement 12.
In the optical system described above, the laser light beam from the laser light source 2 is divided by the diffraction grating 3 into three light beams and therefore there are actually three light beams in both the light path from the diffraction grating 3 to the disc 1 and the light path from the disc 1 to the photodetector 11. These three light beams are caused to impinge on the disc 1 in the form of a main beam 13 for reading the information recorded on the record track T in tracking relation to the same and a couple of auxiliary beams 14E for detecting positional deviation of the main beam 13 from the center of the record track T, as shown in FIG. 2. The auxiliary beams 14E and 14F are located at the positions symmetrical in respect of the location of the main beam 13 in both the directions along and perpendicular to the record track T. The main beam 13 and the auxiliary beams 14E and 14F each reflected from the disc 1 are passed through the lens portion 10, which is composed of a cylindrical lens for detecting the focus condition of the main beam 13, and led to the different light detecting elements in the photodetector 11, respectively.
As shown in FIG. 3, the photodetector 11 comprises four light detecting elements 15A, 15B, 15C and 15D close to one another and two additional light detecting elements 16E and 16F distant from the light detecting elements 15A, 15B, 15C and 15D. The main beam 13 forms a spot on the light detecting elements 15A, 15B, 15C and 15D and the auxiliary beams 14E and 14F form spots on the light detecting elements 16E and 16F, respectively. The light detecting elements 15A, 15B, 15C and 15D produce respective output signals I.sub.A, I.sub.B, I.sub.C and I.sub.D each corresponding to a portion of the beam spot fromed on each of the light detecting elements 15A, 15B, 15C and 15D, and the light detecting elements 16E and 16F produce respective output signals I.sub.E and I.sub.F each corresponding to a portion of the beam spot formed on the light detecting elements 16E and 16F, respectively.
The signal processing circuit arrangement 12 has been previously constituted, for example, as shown in FIG. 4, in which a circuit for obtaining the reproduced information signal is shown together with the tracking servocontrol circuit and the focus servocontrol circuit.
The tracking servocontrol circuit comprises a subtracter 17 which is supplied with the output signals I.sub.E and I.sub.F of the light detecting elements 16E and 16F and produces a tracking error signal which represents positional deviation of the main beam 13 from the center of the record track, that is, a tracking error expressed as I.sub.E -I.sub.F. The tracking servocontrol circuit further comprises a servocontrol amplifier 18 which is supplied with the tracking error signal I.sub.E -I.sub.F and supplies a driving current varying in response to the tracking error signal I.sub.E -I.sub.F to the driving device 8 provided for tracking servocontrol. The driving device 8 is composed of a moving coil, and the object lens 7 is driven in response to the driving current flowing through the moving coil, so that the main beam 13 and the auxiliary beams 14E and 14F are shifted as a group in the direction transverse to the record track. The tracking error signal I.sub.E -I.sub.F varies in polarity and level in response to the positional deviation of the main beam 13 from the center of the record track in the inner side or outer side of the disc 1 and the object lens 7 is so moved that the tracking error signal I.sub.E -I.sub.F becomes zero. Thus, the tracking servocontrol is carried out.
Incidentally, the focus servocontrol circuit comprises an adder 19 for adding the output signal I.sub.A from the light detecting element 15A to the output signal I.sub.B from the light detecting element 15B, an adder 20 for adding the output signal I.sub.C from the light detecting element 15C to the output signal I.sub.D from the light detecting element 15D, a subtracter 21 which is supplied with the outputs from the adders 19 and 20 and produces a focus error signal (I.sub.A +I.sub.B)-(I.sub.C +I.sub.D), and a servocontrol amplifier 22 which is supplied with the focus error signal and supplies a driving current varying in response to the focus error signal to the driving device 9 provided for focus servocontrol.
Further, the output signals I.sub.A +I.sub.B and I.sub.C +I.sub.D of the adders 19 and 20 are supplied to an adder 23 so that the reproduced information signal I.sub.A +I.sub.B +I.sub.C +I.sub.D is obtained from the adder 23.
When the optical reproducing apparatus which employs such an optical system containing the tracking servocontrol circuit as described above is subjected to shocks or vibrations caused by some external force, variations in the mutual positional relation between the disc and the object lens are caused so that the tracking error is increased. In such a case, a situation may result in which the tracking error in respect of the light beams tracing a certain circle of the record track on the disc is suddenly increased by shocks to or vibrations of the optical reproducing apparatus to such an extent that the tracking servocontrol can no longer compensate. An undesirable track shift movement of the light beam may then occur, in which the light beam is shifted to another circle of the record track adjacent to the former circle of the record track and stays thereon, under the condition in which the focus servocontrol is still operative correctly despite the shocks or vibrations.
For the purpose of maintaining the light beams directed to the disc in correct tracking relation to the record track and avoiding the light beam track shift movement when the optical reproducing apparatus is subjected to shocks or vibrations caused by external force, it is advantagious to set the tracking servocontrol circuit so as to have a relatively high gain for control operation. When the gain with which the tracking servocontrol circuit performs the control operation is high, the tracking servocontrol circuit responds sensitively to a slight tracking error in respect of the light beams directed to the disc in order to drive a driving device for controlling, for example, the position of an object lens so as to eliminate the tracking error, and accordingly the tracking error is prevented from being increased excessively by shocks or vibrations affecting the optical reproducing apparatus.
The tracking servocontrol circuit with the gain set to be relatively high is effective for increasing resistance to shocks or vibrations and therefore advantageous for reproducing information from a disc which deos not have any defect such as a blemish on the recorded surface thereof. However, when a disc from which information is reproduced has defects on the recorded surface thereof, the tracking servocontrol circuit with the gain set to be relatively high brings about the following problem.
In the case of the disc having such defects on the recorded surface thereof, the defect would be read out by the auxiliary beams provided for detecting the tracking error as described above and a pulse noise resulting from the defect read out by the auxiliary beam would appear in the tracking error signal. When such a pulse noise appears in the tracking error signal, a pulse current is supplied to the driving device provided for tracking servocontrol in response to the pulse noise. In such a case, if the gain with which the tracking servocontrol circuit performs the control operation is set to be relatively high, the pulse current flowing through the driving device provided for tracking servocontrol in response to the pulse noise becomes extremely large in amplitude so that the tracking servocontrol is put into disorder and therefrore the track shift movement of the light beam on the disc is caused.